drm: exynos: mixer: fix using usleep() in atomic context

This patch fixes calling usleep_range() after taking reg_slock
using spin_lock_irqsave(). The mdelay() is used instead.
Waiting in atomic context is not the best idea in general.
Hopefully, waiting occurs only when Video Processor fails
to reset correctly.

Signed-off-by: Tomasz Stanislawski <t.stanislaws@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>